FIG. 6 is a scheme illustrating the track pattern on a magnetic tape, on which information is recorded/reproduced by an R-DAT, and FIG. 7 is a top view of a usual tape driving system of the R-DAT, in which the reference numeral 1 is an arrow indicating the movement direction of the tape; 2 is an arrow indicating the movement direction of the rotary head; 3 is a reference edge; 4, 4' are selection tracks; 5, 5' are track guards; 6.sub.1, 6.sub.2 are main tracks; 7 is a cassette; 8 is a drum; 9 is a capstan; and 10 represents the tape.
As indicated in the figures, two heads on the drum 8 record two tracks 6.sub.1, 6.sub.2 for every rotation of the drum 8 and inversely, at the reproduction, the two tracks 6.sub.1, 6.sub.2 are reproduced for every rotation of the drum 8. Although here it is supposed that the number of heads is 2, it is only the track pattern indicated in FIG. 6 that is determined by the standard. As indicated in FIG. 7, the tape 10 is wound around the drum 8 over 90.degree..
The diamater of the drum is determined to be d=30 mm, but when it is d=15 mm, which is a half thereof, the winding angle is changed from the prior art winding angle of 90.degree. to 180.degree., as indicated in FIG. 8, and thus the drum may be made more slender. In order to make the R-DAT more convenient for carrying it, it is necessary to make it smaller and inevitable to make it thinner. However, only if the diameter d is changed to 15 mm, as indicated in FIG. 8, although it can be made smaller in the direction perpendicular to the axis 11' of the motor 11, its thickness in the direction parallel to the axis 11' of the drum 8 and the motor 11 cannot be reduced.